I think at their bests Brown wins by using those sneaky counters and body punching to wear down Mayorga. Mayorga isn't tall or particularly slick so he would be the kind of opponent that Brown did well against. Later career Brown gets drawn into a war and it's up for grabs.
Only Tito and DLH beat the fuck out of mayorga. I dont think Brown was in their class. He should win, but el matador is competitive
That's how I see it. Coming right to Simon Brown and getting in a war is playing to his strengths. And, as I remember Mayorga, he needed some space between bursts of activity and Brown wouldn't let him do that.